442 REVIEWS 



county, New York. These are classified as Grenville (Algonkian) 

 rocks, igneous rocks intrusive in the Grenville, and other igneous 

 rocks of doubtful age, possibly in part older than the Grenville rocks. 



The Grenville rocks occur in small disconnected patches sur- 

 rounded by intrusive igneous rocks. Some of them have such position 

 with reference to one another that they seem to represent remnants of 

 what were originally two continuous parallel N. E. to S. W. belts. 

 The characteristic rock of the series is the crystalline marble. This is 

 intricately infolded with quartzose and hornblendic gneisses, and with 

 fine-grained granitic, syenitic, and gabbroic gneisses precisely like 

 gneisses which occur in other areas where no member of the Grenville 

 series is to be found. 



The gneisses of undetermined age include granite, syenite, diorite, 

 and gabbro gneisses, together with intermediate varieties. They 

 occupy a very large area. If all these gneisses are igneous (as is 

 thought probable) there are three possibilities in regard to their age. 



1. They may represent in whole or part a more ancient series than 

 the Grenville. 



2. They may represent a somewhat later series intrusive in the Gren- 

 ville, but older than the great gabbro, syenite, and granite intrusions. 



3. They may represent thoroughly foliated phases of these later 

 intrusions. 



In Dr. Cushing's present judgment they will be found to belong 

 partly under 2 and partly under 3, but more especially the former. 



No rocks have been found in the northern Adirondacks which can 

 be shown to be older than the Grenville series, but in every case in 

 which the relations have been made out, the adjacent rocks show intru- 

 sive contacts with the Grenville rocks. On the other hand, the Gren- 

 ville is a sedimentary series and must have been laid down on some floor. 



Younger than the Grenville rocks and for the most part younger 

 than the doubtful gneisses are a considerable quantity of igneous rocks 

 comprising gabbros (anorthosites) syenites, and granites. These again 

 occupy large areas. 



In the northern portion of the county. Upper Cambrian rocks 

 overlie the pre-Cambrian rocks with unconformity. 



Smyth ' discusses certain features of recent work in the western 



' The Crystalline Rocks of Western Adirondack Region, by C. H. Smyth : Rept. 

 of the New York State Geologist for 1897, published in Fifty-first Ann. Rept. of New 

 York State Museum, Vol. II, 1899, pp. 469-467. 



